<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/create_domain.sgml,v 1.13 2003/04/22 10:08:08 petere Exp $ PostgreSQL documentation --> <refentry id="SQL-CREATEDOMAIN"> <refmeta> <refentrytitle id="sql-createdomain-title">CREATE DOMAIN</refentrytitle> <refmiscinfo>SQL - Language Statements</refmiscinfo> </refmeta> <refnamediv> <refname>CREATE DOMAIN</refname> <refpurpose>define a new domain</refpurpose> </refnamediv> <refsynopsisdiv> <synopsis> CREATE DOMAIN <replaceable class="parameter">domainname</replaceable> [AS] <replaceable class="parameter">data_type</replaceable> [ DEFAULT <replaceable>default_expr</> ] [ <replaceable class="PARAMETER">constraint</replaceable> [ ... ] ] where <replaceable class="PARAMETER">constraint</replaceable> is: [ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ] { NOT NULL | NULL | CHECK (<replaceable class="PARAMETER">expression</replaceable>) } </synopsis> </refsynopsisdiv> <refsect1> <title>Description</title> <para> <command>CREATE DOMAIN</command> creates a new data domain. The user who defines a domain becomes its owner. </para> <para> If a schema name is given (for example, <literal>CREATE DOMAIN myschema.mydomain ...</>) then the domain is created in the specified schema. Otherwise it is created in the current schema. The domain name must be unique among the types and domains existing in its schema. </para> <para> Domains are useful for abstracting common fields between tables into a single location for maintenance. For example, an email address column may be used in several tables, all with the same properties. Define a domain and use that rather than setting up each table's constraints individually. </para> </refsect1> <refsect1> <title>Parameters</title> <variablelist> <varlistentry> <term><replaceable class="parameter">domainname</replaceable></term> <listitem> <para> The name (optionally schema-qualified) of a domain to be created. </para> </listitem> </varlistentry> <varlistentry> <term><replaceable class="PARAMETER">data_type</replaceable></term> <listitem> <para> The underlying data type of the domain. This may include array specifiers. </para> </listitem> </varlistentry> <varlistentry> <term><literal>DEFAULT <replaceable>default_expr</replaceable></literal></term> <listitem> <para> The <literal>DEFAULT</> clause specifies a default value for columns of the domain data type. The value is any variable-free expression (but subqueries are not allowed). The data type of the default expression must match the data type of the domain. If no default value is specified, then the default value is the null value. </para> <para> The default expression will be used in any insert operation that does not specify a value for the column. If a default value is defined for a particular column, it overrides any default associated with the domain. In turn, the domain default overrides any default value associated with the underlying data type. </para> </listitem> </varlistentry> <varlistentry> <term><literal>CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable></literal></term> <listitem> <para> An optional name for a constraint. If not specified, the system generates a name. </para> </listitem> </varlistentry> <varlistentry> <term><literal>NOT NULL</></term> <listitem> <para> Values of this domain are not allowed to be null. </para> </listitem> </varlistentry> <varlistentry> <term><literal>NULL</></term> <listitem> <para> Values of this domain are allowed to be null. This is the default. </para> <para> This clause is only intended for compatibility with nonstandard SQL databases. Its use is discouraged in new applications. </para> </listitem> </varlistentry> <varlistentry> <term><literal>CHECK (<replaceable class="PARAMETER">expression</replaceable>)</literal></term> <listitem> <para> <literal>CHECK</> clauses specify integrity constraints or tests which values of the domain must satisfy. Each constraint must be an expression producing a Boolean result. It should use the name <literal>VALUE</> to refer to the value being tested. </para> <para> Currently, <literal>CHECK</literal> expressions cannot contain subqueries nor refer to variables other than <literal>VALUE</>. </para> </listitem> </varlistentry> </variablelist> </refsect1> <refsect1> <title>Diagnostics</title> <variablelist> <varlistentry> <term><computeroutput>CREATE DOMAIN</computeroutput></term> <listitem> <para> Message returned if the domain was successfully created. </para> </listitem> </varlistentry> </variablelist> </refsect1> <refsect1> <title>Examples</title> <para> This example creates the <type>country_code</type> data type and then uses the type in a table definition: <programlisting> CREATE DOMAIN country_code char(2) NOT NULL; CREATE TABLE countrylist (id integer, country country_code); </programlisting> </para> </refsect1> <refsect1 id="SQL-CREATEDOMAIN-compatibility"> <title>Compatibility</title> <para> The command <command>CREATE DOMAIN</command> conforms to the SQL standard. </para> </refsect1> <refsect1 id="SQL-CREATEDOMAIN-see-also"> <title>See Also</title> <simplelist type="inline"> <member><xref linkend="sql-dropdomain" endterm="sql-dropdomain-title"></member> </simplelist> </refsect1> </refentry> <!-- Keep this comment at the end of the file Local variables: mode: sgml sgml-omittag:nil sgml-shorttag:t sgml-minimize-attributes:nil sgml-always-quote-attributes:t sgml-indent-step:1 sgml-indent-data:t sgml-parent-document:nil sgml-default-dtd-file:"../reference.ced" sgml-exposed-tags:nil sgml-local-catalogs:"/usr/lib/sgml/catalog" sgml-local-ecat-files:nil End: -->